Network Working Group E. Duros
Request for Comments: 3077 UDcast
Category: Standards Track W. Dabbous
INRIA Sophia-Antipolis
H. Izumiyama
N. Fujii
WIDE
Y. Zhang
HRL
March 2001
A Link-Layer Tunneling Mechanism for Unidirectional Links
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document describes a mechanism to emulate full bidirectional
connectivity between all nodes that are directly connected by a
unidirectional link. The "receiver" uses a link-layer tunneling
mechanism to forward datagrams to "feeds" over a separate
bidirectional IP (Internet Protocol) network. As it is implemented
at the link-layer, protocols in addition to IP may also be supported
by this mechanism.
1. Introduction
Internet routing and upper layer protocols assume that links are
bidirectional, i.e., directly connected hosts can communicate with
each other over the same link.
This document describes a link-layer tunneling mechanism that allows
a set of nodes (feeds and receivers, see Section 2 for terminology)
which are directly connected by a unidirectional link to send
datagrams as if they were all connected by a bidirectional link. We
present a generic topology in section 3 with a tunneling mechanism
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RFC 3077 LL Tunneling Mechanism for UDLs March 2001
that supports multiple feeds and receivers. Note, this mechanism is
not designed for topologies where a pair of nodes are connected by 2
unidirectional links in opposite direction.
The tunneling mechanism requires that all nodes have an additional
interface to an IP interconnected infrastructure.
The tunneling mechanism is implemented at the link-layer of the
interface of every node connected to the unidirectional link. The
aim is to hide from higher layers, i.e., the network layer and above,
the unidirectional nature of the link. The tunneling mechanism also
includes an automatic tunnel configuration protocol that allows nodes
to come up/down at any time.
Generic Routing Encapsulation [RFC2784] is suggested as the tunneling
mechanism as it provides a means for carrying IP, ARP datagrams, and
any other layer-3 protocol between nodes.
The tunneling mechanism described in this document was discussed and
agreed upon by the UDLR working group.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
2. Terminology
Unidirectional link (UDL): A one way transmission link, e.g., a
broadcast satellite link.
Receiver: A router or a host that has receive-only connectivity to a
UDL.
Send-only feed: A router that has send-only connectivity to a UDL.
Receive capable feed: A router that has send-and-receive connectivity
to a UDL.
Feed: A send-only or a receive capable feed.
Node: A receiver or a feed.
Bidirectional interface: a typical communication interface that can
send or receive packets, such as an Ethernet card, a modem, etc.
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RFC 3077 LL Tunneling Mechanism for UDLs March 20013. Topology
Feeds and receivers are connected via a unidirectional link. Send-
only feeds can only send data over this unidirectional link, and
receivers can only receive data from it. Receive capable feeds have
both send and receive capabilities.
This mechanism has been designed to work with any topology with any
number of receivers and one or more feeds. However, it is expected
that the number of feeds will be small. In particular, the special
case of a single send-only feed and multiple receivers is among the
topologies supported.
A receiver has several interfaces, a receive-only interface and one
or more additional bidirectional communication interfaces.
A feed has several interfaces, a send-only or a send-and-receive
capable interface connected to the unidirectional link and one or
more additional bidirectional communication interfaces. A feed MUST
be a router.
Tunnels are constructed between the bidirectional interfaces of
nodes, so these interfaces must be interconnected by an IP
infrastructure. In this document we assume that that infrastructure
is the Internet.
Figure 1 depicts a generic topology with several feeds and several
receivers.
Unidirectional Link
---->---------->------------------->------
| | | |
|f1u |f2u |r2u |r1u
-------- -------- -------- -------- ----------
|Feed 1| |Feed 2| |Recv 2| |Recv 1|---|subnet A|
-------- -------- -------- -------- ----------
|f1b |f2b |r2b |r1b |
| | | | |
----------------------------------------------------
| Internet |
----------------------------------------------------
Figure 1: Generic topology
f1u (resp. f2u) is the IP address of the 'Feed 1' (resp. Feed 2)
send-only interface.
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f1b (resp. f2b) is the IP address of the 'Feed 1' (resp. Feed 2)
bidirectional interface connected to the Internet.
r1u (resp. r2u) is the IP address of the 'Receiver 1' (resp. Receiver
2) receive-only interface.
r1b (resp. r2b) is the IP address of the 'Receiver 1' (resp. Receiver
2) bidirectional interface connected to the Internet.
Subnet A is a local area network connected to recv1.
Note that nodes have IP addresses on their unidirectional and their
bidirectional interfaces. The addresses on the unidirectional
interfaces (f1u, f2u, r1u, r2u) will be drawn from the same IP
network. In general the addresses on the bidirectional interfaces
(f1b, f2b, r1b, r2b) will be drawn from different IP networks, and
the Internet will route between them.
4. Problems related to unidirectional links
Receive-only interfaces are "dumb" and send-only interfaces are
"deaf". Thus a datagram passed to the link-layer driver of a
receive-only interface is simply discarded. The link-layer of a
send-only interface never receives anything.
The network layer has no knowledge of the underlying transmission
technology except that it considers its access as bidirectional.
Basically, for outgoing datagrams, the network layer selects the
correct first hop on the connected network according to a routing
table and passes the packet(s) to the appropriate link-layer driver.
Referring to Figure 1, Recv 1 and Feed 1 belong to the same network.
However, if Recv 1 initiates a 'ping f1u', it cannot get a response
from Feed 1. The network layer of Recv 1 delivers the packet to the
driver of the receive-only interface, which obviously cannot send it
to the feed.
Many protocols in the Internet assume that links are bidirectional.
In particular, routing protocols used by directly connected routers
no longer behave properly in the presence of a unidirectional link.
5. Emulating a broadcast bidirectional network
The simplest solution is to emulate a broadcast capable link-layer
network. This will allow the immediate deployment of existing higher
level protocols without change. Though other network structures,
such as NBMA, could also be emulated, a broadcast network is more
generally useful. Though a layer 3 network could be emulated, a
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RFC 3077 LL Tunneling Mechanism for UDLs March 2001
link-layer network allows the immediate use of any other network
layer protocols, and most particularly allows the immediate use of
ARP.
A link-layer tunneling mechanism which emulates bidirectional
connectivity in the presence of a unidirectional link will be
described in the next Section. We first consider the various
communication scenarios which characterize a broadcast network in
order to define what functionalities the link-layer tunneling
mechanism has to perform in order to emulate a bidirectional
broadcast link.
Here we enumerate the scenarios which would be feasible on a
broadcast network, i.e., if feeds and receivers were connected by a
bidirectional broadcast link:
Scenario 1: A receiver can send a packet to a feed (point-to-point
communication between a receiver and a feed).
Scenario 2: A receiver can send a broadcast/multicast packet on the
link to all nodes (point-to-multipoint).
Scenario 3: A receiver can send a packet to another receiver (point-
to-point communication between two receivers).
Scenario 4: A feed can send a packet to a send-only feed (point-to-
point communication between two feeds).
Scenario 5: A feed can send a broadcast/multicast packet on the link
to all nodes (point-to-multipoint).
Scenario 6: A feed can send a packet to a receiver or a receive
capable feed (point-to-point).
These scenarios are possible on a broadcast network. Scenario 6 is
already feasible on the unidirectional link. The link-layer
tunneling mechanism should therefore provide the functionality to
support scenarios 1 to 5.
Note that regular IP forwarding over such an emulated network (i.e.,
using the emulated network as a transit network) works correctly; the
next hop address at the receiver will be the unidirectional link
address of another router (a feed or a receiver) which will then
relay the packet.
Duros, et al. Standards Track [Page 5]

RFC 3077 LL Tunneling Mechanism for UDLs March 20016.1. Tunneling mechanism on the receiver
On the receiver, a datagram is delivered to the link-layer of the
unidirectional interface for transmission (see Figure 2). It is then
encapsulated within a MAC header corresponding to the unidirectional
link. This packet cannot be sent directly over the link, so it is
then processed by the tunneling mechanism.
The packet is encapsulated within an IP header whose destination is
the IP address of a feed bidirectional interface (f1b or f2b). This
destination address is also called the tunnel end-point. The
mechanism for a receiver to learn these addresses and to choose the
feed is explained in Section 7. The type of encapsulation is
described in Section 8.
In all cases the packet is encapsulated, but the tunnel end-point (an
IP address) depends on the encapsulated packet's destination MAC
address. If the destination MAC address is:
1) the MAC address of a feed interface connected to the
unidirectional link (Scenario 1). The datagram is
encapsulated, the destination address of the encapsulating
datagram is the feed tunnel end-point (f1b referring to Figure
2).
2) a MAC broadcast/multicast address (Scenario 2). The datagram
is encapsulated, the destination address of the encapsulating
datagram is the default feed tunnel end-point. See Section 7.4
for further details on the default feed.
3) a MAC address that belongs to the unidirectional network but is
not a feed address (Scenario 3). The datagram is encapsulated,
the destination address of the encapsulating datagram is the
default feed tunnel end-point.
The encapsulated datagram is passed to the network layer which
forwards it according to its destination address. The destination
address is a feed bidirectional interface which is reachable via the
Internet. In this case, the encapsulated datagram is forwarded via
the receiver bidirectional interface (r1b).
6.2. Tunneling mechanism on the feed
A feed processes unidirectional link related packets in two different
ways:
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RFC 3077 LL Tunneling Mechanism for UDLs March 2001
- packets generated by a local application or packets routed as
usual by the IP layer may have to be forwarded over the
unidirectional link (Section 6.2.1)
- encapsulated packets received from another receiver or feed need
tunnel processing (Section 6.2.2).
A feed cannot directly send a packet to a send-only feed over the
unidirectional link (Scenario 4). In order to emulate this type of
communication, feeds have to tunnel packets to send-only feeds. A
feed MUST maintain a list of all other feed tunnel end-points. This
list MUST indicate which are send-only feed tunnel end-points. This
is configured manually at the feed by the local administrator, as
described in Section 7.
6.2.1. Forwarding packets over the unidirectional link
When a datagram is delivered to the link-layer of the unidirectional
interface of a feed for transmission, its treatment depends on the
packet's destination MAC address. If the destination MAC address is:
1) the MAC address of a receiver or a receive capable feed
(Scenario 6). The packet is sent over the unidirectional link.
This is classical "forwarding".
2) the MAC address of a send-only feed (Scenario 4). The packet
is encapsulated and sent to the send-only feed tunnel end-
point. The type of encapsulation is described in Section 8.
3) a broadcast/multicast destination (Scenario 5). The packet is
sent over the unidirectional link. Concurrently, a copy of
this packet is encapsulated and sent to every feed of the list
of send-only feed tunnel end-points. Thus the
broadcast/multicast will reach all receivers and all send-only
feeds.
6.2.2. Receiving encapsulated packets
Feeds listen for incoming encapsulated datagrams on their tunnel
end-points. Encapsulated packets will have been received on a
bidirectional interface, and traversed their way up the IP stack.
They will then enter a decapsulation process (See Figure 2).
Decapsulation reveals the original link-layer packet. Note that this
has not been modified in any way by intermediate routers; in
particular, the original MAC header will be intact.
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Further actions depend on the destination MAC address of the link-
layer packet, which can be:
1) the MAC address of the feed interface connected to the
unidirectional link, i.e., own MAC address (Scenarios 1 and 4).
The packet is passed to the link-layer of the interface
connected to the unidirectional link which can then deliver it
up to higher layers. As a result, the datagram is processed as
if it was coming from the unidirectional link, and being
delivered locally. Scenarios 1 and 4 are now feasible, a
receiver or a feed can send a packet to a feed.
2) a receiver address (Scenario 3). The packet is passed to the
link-layer of the interface connected to the unidirectional
link. It is directly sent over the unidirectional link, to the
indicated receiver. Note, the packet must not be delivered
locally. Scenario 3 is now feasible, a receiver can send a
packet to another receiver.
3) a broadcast/multicast address, this corresponds to Scenarios 2
and 5. We have to distinguish two cases, either (i) the
encapsulated packet was sent from a receiver or (ii) from a
feed (encapsulated broadcast/multicast packet sent to a send-
only feed). These cases are distinguished by examining the
source address of the encapsulating packet and comparing it
with the configured list of feed IP addresses. The action then
taken is:
i) the feed was designated as a default feed by a receiver to
forward the broadcast/multicast packet. The feed is then in
charge of sending the multicast packet to all nodes.
Delivery to all nodes is accomplished by executing all 3 of
the following actions:
- The packet is encapsulated and sent to the list of send-
only feed tunnel end-points.
- Also, the packet is passed to the link-layer of the
interface which forwards it directly over the
unidirectional link (all receivers and receive capable
feeds receive it).
- Also, the link-layer delivers it locally to higher
layers.
Caution: a receiver which sends an encapsulated
broadcast/multicast packet to a default feed will receive
its own packet via the unidirectional link. Correct
filtering as described in [RFC1112] must be applied.
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RFC 3077 LL Tunneling Mechanism for UDLs March 2001
ii) the feed receives the packet and keeps it for local
delivery. The packet is passed to the link-layer of the
interface connected to the unidirectional link which
delivers it to higher layers.
Scenario 2 is now feasible, a receiver can send a
broadcast/multicast packet over the unidirectional link and it
will be heard by all nodes.
7. Dynamic Tunnel Configuration Protocol (DTCP)
Receivers and feeds have to know the feed tunnel end-points in order
to forward encapsulated datagrams (e.g., Scenarios 1 and 4).
The number of feeds is expected to be relatively small (Section 3),
so at every feed the list of all feeds is configured manually. This
list should note which are send-only feeds, and which are receive
capable feeds. The administrator sets up tunnels to all send-only
feeds. A tunnel end-point is an IP address of a bidirectional link
on a send-only feed.
For scalability reasons, manual configuration cannot be done at the
receivers. Tunnels must be configured and maintained dynamically by
receivers, both for scalability, and in order to cope with the
following events:
1) New feed detection.
When a new feed comes up, every receiver must create a tunnel
to enable bidirectional communication with it.
2) Loss of unidirectional link detection.
When the unidirectional link is down, receivers must disable
their tunnels. The tunneling mechanism emulates bidirectional
connectivity between nodes. Therefore, if the unidirectional
link is down, a feed should not receive datagrams from the
receivers. Protocols that consider a link as operational if
they receive datagrams from it (e.g., the RIP protocol
[RFC2453]) require this behavior for correct operation.
3) Loss of feed detection.
When a feed is down, receivers must disable their corresponding
tunnel. This prevents unnecessary datagrams from being
tunneled which might overload the Internet. For instance,
there is no need for receivers to forward a broadcast message
through a tunnel whose end-point is down.
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RFC 3077 LL Tunneling Mechanism for UDLs March 2001
Sequence (16 bit unsigned integer): Random value initialized at boot
time and incremented by 1 every time a value of the HELLO message
is modified.
res (3 bits): Reserved/unused field, MUST be zero.
F (1 bit): bit indicating the type of feed:
0 = Send-only feed
1 = Receive-capable feed
IP Vers (4 bit unsigned integer): IP protocol version of the feed
bidirectional IP addresses (FBIP):
4 = IP version 4
6 = IP version 6
Tunnel Type (8 bit unsigned integer): tunneling protocol supported by
the feed. This value is the IP protocol number defined in
[RFC1700] [iana/protocol-numbers] and their legitimate
descendents. Receivers MUST use this form of tunnel encapsulation
when tunneling to the feed.
47 = GRE [RFC2784] (recommended)
Other protocol types allowing link-layer encapsulation are
permitted. Obtaining new values is documented in [RFC2780].
Nb of FBIP (8 bit unsigned integer): Number of bidirectional IP feed
addresses which are enumerated in the HELLO message
reserved (8 bits): Reserved/unused field, MUST be zero.
Feed BDL IP addr (32 or 128 bits). The bidirectional IP address feed
is the IP address of a feed bidirectional interface (tunnel end-
point) reachable via the Internet. A feed has 'Nb of FBIP' IP
addresses which are operational tunnel end-points. They are
enumerated in preferred order. FBIP1 being the most suitable
tunnel end-point.
7.2. DTCP on the feed: sending HELLO packets
The DTCP protocol runs on top of UDP. Packets are sent to the "DTCP
announcement" multicast address over the unidirectional link on port
HELLO_PORT with a TTL of 1. Due to existing deployments a feed
SHOULD also support the use of the old DTCP announcement address, as
described in Appendix B.
The source address of the HELLO packet is set to the IP address of
the feed interface connected to the unidirectional link. In the rest
of the document, this value is called FUIP (Feed Unidirectional IP
address).
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RFC 3077 LL Tunneling Mechanism for UDLs March 2001
The process in charge of sending HELLO packets fills every field of
the datagram according to the description given in Section 7.1.
As long as a feed is up and running, it periodically announces its
presence to receivers. It MUST send HELLO packets containing a JOIN
command every HELLO_INTERVAL over the unidirectional link.
Referring to Figure 1 in Section 3, Feed 1 (resp. Feed 2) sends HELLO
messages with the FBIP1 field set to f1b (resp. f2b).
When a feed is about to be shut down, or when routing over the
unidirectional link is about to be intentionally interrupted, it is
recommended that feeds:
1) stop sending HELLO messages containing a JOIN command.
2) send a HELLO message containing a LEAVE command to inform
receivers that the feed is no longer performing routing over
the unidirectional link.
7.3. DTCP on the receiver: receiving HELLO packets
Based on the reception of HELLO messages, receivers discover the
presence of feeds, maintain a list of active feeds, and keep track of
the tunnel end-points for those feeds.
For each active feed, and each IP protocol supported, at least the
following information will be kept:
FUIP - feed unidirectional link IP address
FUMAC - MAC address corresponding to the above IP
address
(FBIP1,...,FBIPn) - list of tunnel end-points
tunnel type - tunnel type supported by this feed
Sequence - "Sequence" value from the last HELLO received
from this feed
timer - used to timeout this entry
The FUMAC value for an active feed is needed for the operation of
this protocol. However, the method of discovery of this value is not
specified here.
Initially, the list of active feeds is empty.
When a receiver is started, it MUST run a process which joins the
"DTCP announcement" multicast group and listens to incoming packets
on the HELLO_PORT port from the unidirectional link.
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Upon the reception of a HELLO message, the process checks the version
number of the protocol. If it is different from HELLO_VERSION, the
packet is discarded and the process waits for the next incoming
packet.
After successfully checking the version number further action depends
on the type of command:
- JOIN:
The process verifies if the address FUIP already belongs to the
list of active feeds.
If it does not, a new entry, for feed FUIP, is created and added
to the list of active feeds. The number of feed bidirectional IP
addresses to read is deduced from the 'Nb of FBID' field. These
tunnel end-points (FBIP1,...,FBIPn) can then be added to the new
entry. The tunnel Type and Sequence values are also taken from
the HELLO packet and recorded in the new entry. A timer set to
HELLO_LEAVE is associated with this entry.
If it does, the sequence number is compared to the sequence number
contained in the previous HELLO packet sent by this feed. If they
are equal, the timer associated with this entry is reset to
HELLO_LEAVE. Otherwise all the information corresponding to FUIP
is set to the values from the HELLO packet.
Referring to Figure 1 in Section 3, both receivers (recv 1 and
recv 2) have a list of active feeds containing two entries: Feed 1
with a FUIP of f1u and a list of tunnel end-points (f1b); and Feed
2 with a FUIP of f2u and a list of tunnel end-points (f2b).
- LEAVE:
The process checks if there is an entry for FUIP in the list of
active feeds. If there is, the timer is disabled and the entry is
deleted from the list. The LEAVE message provides a means of
quickly updating the list of active feeds.
A timeout occurs for either of two reasons:
1) a feed went down without sending a LEAVE message. As JOIN
messages are no longer sent from this feed, a timeout occurs at
HELLO_LEAVE after the last JOIN message.
2) the unidirectional link is down. Thus no more JOIN messages
are received from any of the feeds, and they will each timeout
independently. The timeout of each entry depends on its
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RFC 3077 LL Tunneling Mechanism for UDLs March 2001
individual HELLO_LEAVE value, and when the last JOIN message
was sent by that feed, before the unidirectional link went
down.
In either case, bidirectional connectivity can no longer be ensured
between the receiver and the feed (FUIP): either the feed is no
longer routing datagrams over the unidirectional link, or the link is
down. Thus the associated entry is removed from the list of active
feeds, whatever the cause. As a result, the list only contains
operational tunnel end-points.
The HELLO protocol provides receivers with a list of feeds, and a
list of usable tunnel end-points (FBIP1,..., FBIPn) for each feed.
In the following Section, we describe how to integrate the HELLO
protocol into the tunneling mechanism described in Sections 6.1 and
6.2.
7.4. Tunneling mechanism using the list of active feeds
This Section explains how the tunneling mechanism uses the list of
active feeds to handle datagrams which are to be tunneled. Referring
to Section 6.1, it shows how feed tunnel end-points are selected.
The choice of the default feed is made independently at each
receiver. The choice is a matter of local policy, and this policy is
out of scope for this document. However, as an example, the default
feed may be the feed that has the lowest round trip time to the
receiver.
When a receiver sends a packet to a feed, it must choose a tunnel
end-point from within the FBIP list. The 'preferred FBIP' is
generally FBIP1 (Section 7.1). For various reasons, a receiver may
decide to use a different FBIP, say FBIPi instead of FBIP1, as the
tunnel end-point. For example, the receiver may have better
connectivity to FBIPi. This decision is taken by the receiver
administrator.
Here we show how the list of active feeds is involved when a receiver
tunnels a link-layer packet. Section 6.1 listed the following cases,
depending on whether the MAC destination address of the packet is:
1) the MAC address of a feed interface connected to the
unidirectional link: This is TRUE if the address matches a
FUMAC address in the list of active feeds. The packet is
tunneled to the preferred FBIP of the matching feed.
2) the broadcast address of the unidirectional link or a multicast
address:
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RFC 3077 LL Tunneling Mechanism for UDLs March 2001
This is determined by the MAC address format rules, and the
list of active feeds is not involved. The packet is tunneled
to the preferred FBIP of the default feed.
3) an address that belongs to the unidirectional network but is
not a feed address:
This is TRUE if the address is neither broadcast nor multicast,
nor found in the list of active feeds. The packet is tunneled
to the preferred FBIP of the default feed.
In all cases, the encapsulation type depends on the tunnel type
required by the feed which is selected.
7.5. Constant definitions
DTCP_VERSION is 1.
HELLO_INTERVAL is 5 seconds.
"DTCP announcement" multicast group is 224.0.0.36, assigned by IANA.
HELLO_PORT is 652. It is a reserved system port assigned by IANA, no
other traffic must be allowed.
HELLO_LEAVE is 3*Interval, as advertised in a HELLO packet, i.e., 15
seconds if the default HELLO_INTERVAL was advertised.
8. Tunnel encapsulation format
The tunneling mechanism operates at the link-layer and emulates
bidirectional connectivity amongst receivers and feeds. We assume
that hardware connected to the unidirectional link supports broadcast
and unicast MAC addressing. That is, a feed can send a packet to a
particular receiver using a unicast MAC destination address or to a
set of receivers using a broadcast/multicast destination address.
The hardware (or the driver) of the receiver can then filter the
incoming packets sent over the unidirectional links without any
assumption about the encapsulated data type.
In a similar way, a receiver should be capable of sending unicast and
broadcast MAC packets via its tunnels. Link-layer packets are
encapsulated. As a result, after decapsulating an incoming packet,
the feed can perform link-layer filtering as if the data came
directly from the unidirectional link (See Figure 2).
Generic Routing Encapsulation (GRE) [RFC2784] suits our requirements
because it specifies a protocol for encapsulating arbitrary packets,
and allows use of IP as the delivery protocol.
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The feed's local administrator decides what encapsulation it will
demand that receivers use, and sets the tunnel type field in the
HELLO message appropriately. The value 47 (decimal) indicates GRE.
Other values can be used, but their interpretation must be agreed
upon between feeds and receivers. Such usage is not defined here.
8.1. Generic Routing Encapsulation on the receiver
A GRE packet is composed of a header in which a type field specifies
the encapsulated protocol (ARP, IP, IPX, etc.). See [RFC2784] for
details about the encapsulation. In our case, only support for the
MAC addressing scheme of the unidirectional link MUST be implemented.
A packet tunneled with a GRE encapsulation has the following format:
the delivery header is an IP header whose destination is the tunnel
end-point (FBIP), followed by a GRE header specifying the link-layer
type of the unidirectional link. Figure 4 presents the entire
encapsulated packet.
----------------------------------------
| IP delivery header |
| destination addr = FBIP |
| IP proto = GRE (47) |
----------------------------------------
| GRE Header |
| type = MAC type of the UDL |
----------------------------------------
| Payload packet |
| MAC packet |
----------------------------------------
Figure 4: Encapsulated packet
9. Issues9.1. Hardware address resolution
Regardless of whether the link is unidirectional or bidirectional, if
a feed sends a packet over a non-point-to-point type network, it
requires the data link address of the destination. ARP [RFC826] is
used on Ethernet networks for this purpose.
The link-layer mechanism emulates a bidirectional network in the
presence of an unidirectional link. However, there are asymmetric
delays between every (feed, receiver) pair. The backchannel between
a receiver and a feed has varying delays because packets go through
the Internet. Furthermore, a typical example of a unidirectional
link is a GEO satellite link whose delay is about 250 milliseconds.
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RFC 3077 LL Tunneling Mechanism for UDLs March 2001
Because of long round trip delays, reactive address resolution
methods such as ARP [RFC826] may not work well. For example, a feed
may have to forward packets at high data rates to a receiver whose
hardware address is unknown. The stream of packets is passed to the
link-layer driver of the feed send-only interface. When the first
packet arrives, the link-layer realizes it does not have the
corresponding hardware address of the next hop, and sends an ARP
request. While the link-layer is waiting for the response (at least
250 ms for the GEO satellite case), IP packets are buffered by the
feed. If it runs out of space before the ARP response arrives, IP
packets will be dropped.
This problem of address resolution protocols is not addressed in this
document. An ad-hoc solution is possible when the MAC address is
configurable, which is possible in some satellite receiver cards. A
simple transformation (maybe null) of the IP address can then be used
as the MAC address. In this case, senders do not need to "resolve"
an IP address to a MAC address, they just need to perform the simple
transformation.
9.2. Routing protocols
The link-layer tunneling mechanism hides from the network and higher
layers the fact that feeds and receivers are connected by a
unidirectional link. Communication is bidirectional, but asymmetric
in bandwidths and delays.
In order to incorporate unidirectional links in the Internet, feeds
and receivers might have to run routing protocols in some topologies.
These protocols will work fine because the tunneling mechanism
results in bidirectional connectivity between all feeds and
receivers. Thus routing messages can be exchanged as on any
bidirectional network.
The tunneling mechanism allows any IP traffic, not just routing
protocol messages, to be forwarded between receivers and feeds.
Receivers can route datagrams on the Internet using the most suitable
feed or receiver as a next hop. Administrators may want to set the
metrics used by their routing protocols in order to reflect in
routing tables the asymmetric characteristics of the link, and thus
direct traffic over appropriate paths.
Feeds and receivers may implement multicast routing and therefore
dynamic multicast routing can be performed over the unidirectional
link. However issues related to multicast routing (e.g., protocol
configuration) are not addressed in this document.
Duros, et al. Standards Track [Page 18]

RFC 3077 LL Tunneling Mechanism for UDLs March 20019.3. Scalability
The DTCP protocol does not generate a lot of traffic whatever the
number of nodes. The problem with a large number of nodes is not
related to this protocol but to more general issues such as the
maximum number of nodes which can be connected to any link. This is
out of scope of this document.
10. IANA Considerations
IANA has reserved the address 224.0.0.36 for the "DTCP announcement"
multicast address as defined in Section 7.
IANA has reserved the udp port 652 for the HELLO_PORT as defined in
Section 7.
11. Security Considerations
Many unidirectional link technologies are characterised by the ease
with which the link contents can be received. If sensitive or
valuable information is being sent, then link-layer security
mechanisms are an appropriate measure. For the UDLR protocol itself,
the feed tunnel end-point addresses, sent in HELLO messages, may be
considered sensitive. In such cases link-layer security mechanisms
may be used.
Security in a network using the link-layer tunneling mechanism should
be relatively similar to security in a normal IPv4 network. However,
as the link-layer tunneling mechanism requires the use of tunnels, it
introduces a potential for unauthorised access to the service. In
particular, ARP and IP spoofing are potential threats because nodes
may not be authorised to tunnel packets. This can be countered by
authenticating all tunnels. The authenticating mechanism is not
specified in this document, it can take place either in the delivery
IP protocol (e.g., AH[RFC2402]) or in an authentication protocol
integrated with the tunneling mechanism.
At a higher level, receivers may not be authorised to provide routing
information even though they are connected to the unidirectional
link. In order to prevent unauthorised receivers from providing fake
routing information, routing protocols running on top of the link-
layer tunneling mechanism MUST use authentication mechanisms when
available.
Duros, et al. Standards Track [Page 19]

RFC 3077 LL Tunneling Mechanism for UDLs March 2001
Appendix A: Conformance and interoperability
This document describes a mechanism to emulate bidirectional
connectivity between nodes that are directly connected by a
unidirectional link. Applicability over a variety of equipment and
environments is ensured by allowing a choice of several key system
parameters.
Thus in order to ensure interoperability of equipment it is not
enough to simply claim conformance with the mechanism defined here.
A usage profile for a particular environment will require the
definition of several parameters:
- the MAC format used
- the tunneling mechanism to be used (GRE is recommended)
- the "tunnel type" indication if GRE is not used
For example, a system might claim to implement "the link-layer
tunneling mechanism for unidirectional links, using IEEE 802 LLC, and
GRE encapsulation for the tunnels."
Appendix B: DTCP announcement address transition plan
Some older receivers listen for DTCP announcements on the 224.0.1.124
multicast address (the "old DTCP announcement" address). In order to
support such legacy receivers, feeds SHOULD be configurable to send
all announcements simultaneously to both the "DTCP announcement"
address, and the "old DTCP announcement" address. The default
setting is to send announcements to just the "DTCP announcement"
address.
In order to encourage the transition plan, the "old" feeds MUST be
updated to send DTCP announcements as defined in this section. The
number of "old" feeds originally deployed is relatively small and
therefore the update should be fairly easy. "New" receivers only
support "new" feeds, i.e., they listen to DTCP announcements on the
"DTCP announcement" address.
Duros, et al. Standards Track [Page 21]

RFC 3077 LL Tunneling Mechanism for UDLs March 2001
Full Copyright Statement
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Duros, et al. Standards Track [Page 25]